Field of the Invention
The present invention relates in general to speed changing apparatus and, in particular, to large-ratio speed changing apparatus. More particularly, the present invention relates to a large-ratio strain wave gearing speed changing apparatus for both speed reduction and speed increase applications with improved power capacity and service life.
Description of the Related Art
Speed changing is indispensable. Frequently a prime mover must work at high rotational speeds for optimized efficiency while the load it drives must run at tenths or even hundredths its speed. One way to obtain such large speed reduction ratio is to use a cascade of individual reducers of smaller ratio but each at its best efficiency.
However, this cascaded speed reduction suffers in overall speed-changing efficiency due to its nature of having the entire load passing successively through each and every reducer stage of the cascade. The arrangement is also bulky for the obvious reason that each stage in the cascade must be fully rated to handle one hundred percent of the total power delivered by the prime mover.
One type of “single-stage” reducers currently used widely is the cycloidal drive manufactured by Sumitomo Heavy Industries, Ltd. of Tokyo, Japan. Although relatively compact for speed-changing ratios ranging from tens to more than one hundred, the single-stage drive is, essentially, one cycloidal gearing half stage followed by an off-axis power extraction half stage.
FIG. 1 schematically illustrates the configuration of such a cycloidal speed reducer in cross section. The conventional device in FIG. 1 has a fixed ring gear 11 and a shaped planet element 12, sometimes a shaped disc or sometimes simply a gear. The planet element 12 engages with and moves inside the ring gear 11 epicyclically, or, hypocyclically in the conventional sense. The two have an as-small-as-possible difference in their working pitch diameters, or tooth numbers for that matter.
For the off-axis power extraction stage, a disc 13 is fixed to the planet element 12 coaxially on their axis 19 and has a number of round holes 17 to allow for engagement by a corresponding number of roller pins 18 planted on the plate 14. In many occasions the holes 17 are formed directly in the element 12, sparing the use of disc 13. This round plate 14 is coupled to the output shaft 16 of the drive and is centered on the central axis 10 of the device. This “power extraction” arrangement allows the drive to deliver a speed-reduction ratio of −K/i, wherein K is the pitch diameter of the planet element 12 and i is the difference between the pitch diameters of elements 11 and 12. In a typical example wherein the ring gear 11 has 80 teeth and a gear version of the planet element 12 has 79 (K=80 mm and i=1 mm using module 1 metric gears), the ratio is −80 when mechanical power is transmitted by the device via the input at shaft 15.
FIG. 2 schematically illustrates the off-axis power extraction coupling used for the prior art cycloidal drive of FIG. 1. At any given time, only one of the typically eight or more pin-roller and cycloidal disc hole engagements is transmitting torque fully. For example, with the angular position of the relative offset and with the direction of rotation as shown, only the pair of pin roller 18C and hole 17C is transmitting power fully for the device.
This is obvious as the edge of the hole 17C of the driving disc 13 that is in contact with the pin roller 18C of the driven plate 14 must be behind the roller 18C along the direction of rotation. In this sense pin roller and hole pairs identified by rollers RB and RD are partially working to transmit power because of the location of their contact points relative to the direction of rotation of the disc 13 and plate 14. In the same sense, the pin-roller and hole pair 18G and 17G is not working at all because the pin roller 18G, the driven, travels behind its contact point with its hole 17G, the driver.
Conventional cycloidal drives rely on a synchronizing engagement between two elements (gears) of different pitch diameter with offset axes. But this is not an optimized mechanism due to low utilization: Of all eight pin/hole pairs shown in FIG. 2, half (four or even five depending on the angular position) of them are not in the position to drive the load. Of the other half, only one can be in a full-effort position to drive the load, the other three are in their partial effort. With limitations such as these, cycloidal drives achieve typically less than 80 percent efficiency under normal load conditions.
Further, to achieve a speed reduction ratio of K, a cycloidal drive requires a fixed ring gear of K+1 teeth. For large ratio, the large ring gear number makes the drive bulky if the rated torque is substantial therefore the teeth must be sufficiently robust—in size. In other words, compactness of the cycloidal drive places a limitation on the torque and power rating of the drive.
Due to advantages such as no backlash, compactness and simple construction, another type of large-ratio reducer widely used in precision and aerospace applications is the harmonic drive manufactured by Harmonic Drive Systems Inc. of Tokyo, Japan. Operating the basic concept known as strain wave gearing, harmonic drive is relatively low in available power rating. The drive also delivers typically less than 60 percent efficiency under normal load because its spline element flexes all the time as the drive operates to transmit mechanical power.
FIG. 1A schematically illustrates the configuration of such a strain wave gearing speed reducer in cross section. The conventional device in FIG. 1A has a fixed circular spine 111 and a flex spline 112. The flex spline 112 engages with and moves inside the circular spine 111 flexingly when driven by the wave generator 115E through input shaft 115 of the drive. In the illustrated example of FIG. 1A, with the flex spline 112 having a tooth number of K and the circular spline K+i, the speed reduction ratio of the device at the output shaft 116 is −K/i.
Essentially the same as in the case of a cycloidal drive, for a strain wave gearing device to have large speed changing ratio, the two spline components must have an as-small-as-possible difference in their respective working tooth numbers. Although physically different in construction compared to cycloidal drives, conventional strain wave gearing speed changing devices suffer the same drawbacks in terms of characteristics such as power rating and power-to-weight ratio described above.
In addition to large-ratio speed reducers there are also the need to increase a slow input speed to an output up to tens, hundreds of times or more faster. Speed increasing is the opposite of reducing in terms of speed change ratio but is also important in many applications.